This invention relates generally to the field of digitally controlled printing devices, and in particular to continuous ink jet printers wherein a liquid ink stream breaks into droplets, some of which are selectively deflected.
Traditionally, digitally controlled color printing capability is accomplished by one of two technologies. The first technology, commonly referred to as xe2x80x9cdrop-on-demandxe2x80x9d ink jet printing, provides ink droplets for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle, thus helping to keep the nozzle clean.
With piezoelectric actuators, an electric field is applied to a piezoelectric material possessing properties that create a mechanical stress in the material causing an ink droplet to be expelled. Piezoelectric actuators, such as that disclosed in U.S. Pat. No. 5,224,843, issued to vanLintel on Jul. 6, 1993, have a piezoelectric crystal in an ink fluid channel that flexes when an electric current flows through it forcing an ink droplet out of a nozzle. In a bubble jet printer, ink in a channel of a printhead is heated, creating a bubble which increases internal pressure ejecting an ink droplet out of a nozzle of the printhead. The bubble then collapses as the heating element cools, and the resulting vacuum draws fluid from a reservoir to replace ink that was ejected from the nozzle.
The second technology, commonly referred to as xe2x80x9ccontinuous streamxe2x80x9d or xe2x80x9ccontinuousxe2x80x9d ink jet printing, uses a pressurized ink source which produces a continuous stream of ink droplets. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the ink droplets are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or disposed of. When print is desired, the ink droplets are not deflected and allowed to strike a print media. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism.
Typically, continuous ink jet printing devices are faster than drop-on-demand devices and produce higher quality printed images and graphics. However, each color printed requires an individual droplet formation, deflection, and capturing system.
Conventional continuous ink jet printers utilize electrostatic charging devices and deflector plates, they require many components and large spatial volumes in which to operate. This results in continuous ink jet printheads and printers that are complicated, have high energy requirements, are difficult to manufacture, and are difficult to control.
U.S. Pat. No. 3,709,432, issued to Robertson on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniformly spaced ink droplets through the use of transducers. The lengths of the filaments before they break up into ink droplets are regulated by controlling the stimulation energy supplied to the transducers, with high amplitude stimulation resulting in short filaments and low amplitudes resulting in long filaments. A flow of air is generated across the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air flow affects the trajectories of the filaments before they break up into droplets more than it affects the trajectories of the ink droplets themselves. By controlling the lengths of the filaments, the trajectories of the ink droplets can be controlled, or switched from one path to another. As such, some ink droplets may be directed into a catcher while allowing other ink droplets to be applied to a receiving member. While this method does not rely on electrostatic means to affect the trajectory of droplets it does rely on the precise control of the break off points of the filaments and the placement of the air flow intermediate to these break off points. Such a system is difficult to control and to manufacture. Furthermore, the physical separation or amount of discrimination between the two droplet paths is small, further adding to the difficulty of control and manufacture.
U.S. Pat. No. 4,190,844, issued to Taylor on Feb. 26, 1980, discloses a continuous ink jet printer having a first pneumatic deflector for deflecting non-printed ink droplets to a catcher and a second pneumatic deflector for oscillating printed ink droplets. A printhead supplies a filament of working fluid that breaks into individual ink droplets. The ink droplets are then selectively deflected by a first pneumatic deflector, a second pneumatic deflector, or both. The first pneumatic deflector is an xe2x80x9con/offxe2x80x9d or an xe2x80x9copen/closedxe2x80x9d type having a diaphragm that either opens or closes a nozzle depending on one of two distinct electrical signals received from a central control unit. This determines whether the ink droplet is to be printed or non-printed. The second pneumatic deflector is a continuous type having a diaphragm that varies the amount a nozzle is open depending on a varying electrical signal received the central control unit. This oscillates printed ink droplets so that characters may be printed one character at a time. If only the first pneumatic deflector is used, characters are created one line at a time, being built up by repeated traverses of the printhead.
While this method does not rely on electrostatic means to affect the trajectory of droplets it does rely on the precise control and timing of the first (xe2x80x9copen/closedxe2x80x9d) pneumatic deflector to create printed and non-printed ink droplets. Such a system is difficult to manufacture and accurately control, and unfortunately, such printing methods require a separate pneumatic deflector for each nozzle in the printhead. Since such deflectors are relatively slow in action, the printing speed is low relative to current, commercial ink jet systems. Furthermore, the physical separation or amount of discrimination between the two droplet paths is erratic due to the precise timing requirements increasing the difficulty of controlling printed and non-printed ink droplets resulting in poor ink droplet trajectory control.
Additionally, using two pneumatic deflectors complicates construction of the printhead and requires more components. The additional components and complicated structure require large spatial volumes between the printhead and the media, increasing the ink droplet trajectory distance. Increasing the distance of the droplet trajectory decreases droplet placement accuracy and affects the print image quality. Again, there is a need to minimize the distance the droplet must travel before striking the print media in order to insure high quality images. Pneumatic operation requiring the air flows to be turned on and off is necessarily slow in that an inordinate amount of time is needed to perform the mechanical actuation as well as time associated with the settling any transients in the air flow.
U.S. Pat. No. 6,079,821, issued to Chwalek et al. on Jun. 27, 2000, discloses a continuous ink jet printer that uses actuation of asymmetric heaters to create individual ink droplets from a filament of working fluid and deflect those ink droplets. A printhead includes a pressurized ink source and an asymmetric heater operable to form printed ink droplets and non-printed ink droplets. Printed ink droplets flow along a printed ink droplet path ultimately striking a print media, while non-printed ink droplets flow along a non-printed ink droplet path ultimately striking a catcher surface. Non-printed ink droplets are recycled or disposed of through an ink removal channel formed in the catcher. While the ink jet printer disclosed in Chwalek et al. works extremely well for its intended purpose, using a heater to create and deflect ink droplets increases the energy and power requirements of this device.
U.S. patent application Ser. Nos. 09/750,946 and 09/751,232 disclose the use of an air stream to separate ink drops of a plurality of volumes into spatially differing trajectories. Non-imaging droplets, having one grouping of volumes, are not permitted to reach the image receiver, while imaging droplets having a significantly different range of volumes are permitted to make recording marks on the receiver. While printheads employing the invention described in these disclosures work well, there is a certain distance from the printhead that is required for drop formation to be complete. In these printheads, initial jet breakup is caused by temperature changes due to heater activation by electrical pulses. Following the initial fluid breakup, larger drops are created through the coalescence of smaller drops and fluidic strings, and this coalescence distance is a function of fluid and thermal properties (e.g., surface tension, viscosity, thermal conductivity, etc.) as well as the operating conditions such as ink pressure and drop velocity. Generally, the separation airstream cannot be applied to the droplet stream until the desired drop formation has taken place. A method for addressing this problem and improving the drop formation was disclosed in application U.S. Ser. No. 09/910,097, filed Jul. 20, 2001 by Jeanmaire et al., whereby a short pre-pulse of heater activation is used to shorten the droplet coalescence time.
Once the droplets are formed, droplet streams consisting of a plurality of drop sizes transverse the gas separation means on the way to either a ink catcher or the print medium. Unfortunately, some unintended merging of ink droplets may occur due to slightly different droplet velocities in the ink stream containing droplets of differing volumes. Merging of xe2x80x9cprintingxe2x80x9d with other xe2x80x9cprintingxe2x80x9d or xe2x80x9cnon-printingxe2x80x9d droplets while the gas separation force is being applied results in printing droplet paths which are no longer correct. This can either result in misplaced drops on the print medium, drops of incorrect size landing on the print medium, or droplets which fail to reach the print medium.
For this reason, it can be seen that there is a opportunity to provide a modified inkjet printhead and printer of simple construction having simple control of individual ink droplets with more constant relative velocities between droplets of differing volumes. The range over which the gas separation force can be applied can be increased to take advantage of physical designs of gas flow generators which have optimum uniformity characteristics. In this manner, the quality of printing is increased by improving drop placement on the print medium, while retaining the low energy and power consumption advantage of the printing method described above.
In accordance with a feature of the present invention, an apparatus produces a stream of fluid droplets of at least two types, the droplets of one of the types being of greater fluid volume than the droplets of the other type. A droplet forming mechanism is actuatable by a series of energy pulses to create a series of one or more droplets, the first of the droplets of each series being of the one type, and any droplets subsequent to the first of the droplets of each series being of the other type. A controller applies the series of energy pulses to the droplet forming mechanism such that a pulse associated with droplets of the one type has a predetermined energy and pulses associated with the droplets of the other type have energy substantially greater than the predetermined energy.
The energy of the pulses associated with droplets of the other type is about 5% to about 300% greater than the predetermined energy. Preferably, the energy of the pulses associated with droplets of the other type is between about 10% and about 100% greater than the predetermined energy.